It will be appreciated that the self-generated gas pressure existing between sealing members can equally apply to assemblies which are both rotating but at different speeds as, for example, when sealing the annular spaces between the rotating assemblies of multi-shaft gas turbine engines. The environment surrounding such engines is too hot to allow the use of flexible polymer based sealing materials. In such situations one well-known method is to employ non-contacting sealing paths either in the form of labyrinths or closely positioned face-to-face members, either singly or in combination.
For a typical gas turbine annular seal of mean diameter=914 mm. (36 inches), the gap between the annular face sealing members would be in the range 0.038 mm. to 0.076 mm. (0.0015 to 0.0030 inches) in order to keep the gas leakage across the seal to within acceptable performance limits. In gas turbine engines where the high running temperatures cause relatively large movements and distortions of adjacent components, one common method of achieving the close face gap tolerance between the static and dynamic components of the seal is to provide axial movement controlled by spring members, or by pressurised gas acting on an annular area, to one of the components and to feed a supply of pressurised gas into the annular space between the static and dynamic members. The gas forms a thin aerostatic film between the static and rotating members of the seal assembly.
The leakage performance of such gas sealing assemblies can be enhanced in some cases by providing an additional gas labyrinth flow path or a single axially orientated restricted path (sometimes known as an air dam) and placing this restricted flow path in series with the radially orientated aerostatic flow path.